The purpose of this study was to test a virtual case-based Simulated Diabetes Education (SimDE) intervention developed to teach primary care residents how to manage diabetes. Nineteen primary care residency programs, with 341 volunteer residents in all post-graduate years (PGY), were randomly assigned to a SimDE intervention group or control group (CG). The web-based interactive educational intervention tested utilized computerized virtual patients who responded to provider actions through programmed simulation models. Eighteen distinct learning cases (L cases) were assigned to SimDE residents over 6 months. Impact was assessed using performance on 4 virtual assessment cases (A-cases), an objective knowledge test, and pre-post changes in self-assessed diabetes knowledge and confidence. Group comparisons were analyzed using generalized linear mixed models, controlling for clustering of residents within residency programs and differences in baseline knowledge. The percentage of residents appropriately achieving A-case composite clinical goals for glucose, blood pressure, and lipids was: A-Case 1, SimDE=21.2%, CG=1.8%, P=.002; A-Case 2, SimDE=15.7%, CG = 4.7%, P=.02; A-Case 3, SimDE=48.0%, CG=10.4%, P<.001; A-Case 4, SimDE=42.1%, CG=18.7%, P=.004. The mean knowledge score and pre-post changes in self-assessed knowledge and confidence were significantly better for SimDE group than CG. Virtual case-based SimDE enhanced measures of resident performance, improved objective and subjective knowledge scores, and raised self-confidence in managing patients with diabetes. Broader use of such virtual diabetes education using simulation models may be warranted to improve the much needed diabetes care capabilities of the future healthcare workforce.